Because they frequently feel the need to speak about their fetish. They also believe it's normal and common.
It's like walking into a movie theater, enjoying a film, and then suddenly the autistic kid in the front row starts mouth-breathing loudly and speaking loudly about unrelated trivia.
It's just annoying, because those without the fetish not only totally don't connect in any way, it's disruptive and for many an unpleasant topic due to probable disgust.
I mean, most foot fetish guys here seem... not with ill-intent, so I don't hate 'em, but all the above still holds true.